AVIA Note: Subwoofer Calibration

I've been reminded that the subwoofer calibration in AVIA is more complex to use than VE and thus more time consuming to recommend. I think it's time to revisit the reason and advantages afforded by the approach chosen.
AVIA intentionally carries its low bass test signals on each of the five main channels independently instead of the LFE channel for the express purpose of making main channel bass reproduction more accurate. Understanding why this is important means knowing the difference between three different concepts.
a. The signal going to the subwoofer
b. The low bass which is on each main audio channel (left front, center, right front, left surround, right surround).
c. The low bass on the LFE channel.
Most newbies (and I think a lot of pretty well experienced home theater buffs) get confused and think that the subwoofer is used purely for LFE when it actually should also be used to help the other channels reproduce the low bass in the main channels. Indeed, it is possible to have low freq effects carried completely on the mains and nothing on the optional LFE channel.
AVIA is designed to get the highest fidelity out of the main channels - the channels which carry the vast preponderance of sound. By placing the low bass test information solely on the main channels, one is able to isolate and check the bass management of each channel and ensure that all are being appropriately handled in a system.
The setting of speaker size and bass management tremendously affects how bass is handled for each channel. Speakers set to "small" have their low bass routed to the bass output channel (s) which can be either just the subwoofer , the "large" speakers or some combination of subwoofer and large speaker depending on how the receiver is set up.
We highly recommend setting all speakers to "small" because the very very low bass content down to the 20's Hz simply is not as well reproduced by most main speakers as a dedicated sub. Even if one has powered subs inside the main speakers, room placement of those speakers is rarely if ever the best location for bass reproduction. By setting the speakers to "small" you give each speaker a chance to excel in what they do best.
If a speaker is set to "large" its low bass content will not be sent to the subwoofer output. Remember this! AVIA's subwoofer tests are on the main channels. If a particular channel is set to large, then that channel's AVIA test tone is not routed to the subwoofer (just like low bass on that channel is not routed to the subwoofer). This means that the AVIA subwoofer test behaves exactly the same way as live material to be played back on that channel and you can examine that behavior to learn how real material behaves. This also means that playing AVIA subwoofer tests for a channel set to large produces nothing on the sub.
Speakers set to "small" have their low bass routed to either the sub and/or "large" speakers depending on how you have set up the processor to handle bass. AVIA subwoofer tones in channels set "small" behave exactly the way regular bass material in that channel does.
So keep track of how you set your speakers and bass management. AVIA's signals will be routed exactly that way.
It is frequent for people to find that low bass in their various channels is not routed in ways they previously expected. Listen to what the AVIA tones do and you'll discover what is actually happening.
During calibration of your subwoofer level, you usually don't have an ability to independently adjust the strength of bass coming from each main channel. This can mean that you find the SPL reading of subwoofer tests vary depending on which channel is being tested. The most common reason for this is a difference in how the bass is being processed (one channel is large while the other is small). Since you can't independently adjust each, it is reasonable to either average for the front three channels or simply concentrate on getting the front channel right since that one carries the most work in a movie soundtrack.
If nothing comes out of the subwoofer during AVIA tests, it's most likely because all the speakers are set to "large." That prevents routing of low bass in all the channels to the subwoofer. Only the LFE channel would play out of the subwoofer.
The LFE channel is normally not independently calibrated. Some systems allow you to set things between -10 dB and 0 dB (normal). Ordinarily, leaving the LFE at 0 dB gives good results once the main channel bass has been calibrated.
If LFE must be independently calibrated, use the Low Freq 6 Channel Sweep in AVIA (Title 6 chapter 29). This test has a discrete LFE channel signal to verify that LFE level is correct relative to the other 5 channels of bass content. All should read the same SPL.
(For the overly knowledgeable ... the LFE signal is already pre-compensated -10 dB in intensity so it yields the same SPL as the other channels if LFE gain is at 0 dB. No need to worry about the 10 dB playback boost, it's been accounted for)
Getting the subwoofer to main channel sound pressure levels correct is only the beginning. There is a critical overlap frequency range for each channel which is around the crossover frequency. At those frequencies both the main and subwoofer are involved in creating sound simultaneously. Proper blending of the mains and subwoofer requires that the sound from both subwoofer and main speakers be in phase. Otherwise the main speaker and sub cancel each other out in that frequency range and one creates a hole in overall system frequency response.
AVIA allows detailed testing of response at the frequency crossover range for both phase adjustment and frequency selection. This can be done for each of the main channels, but usually getting it right for the center speaker is sufficient if you have already correctly positioned the left and right front speakers to be in phase with the center speaker.
The low frequency phase tests in AVIA have acoustic energy throughout the usual bandpass region. If the subwoofer is out of phase with the speaker channel being tested, partial cancellation of some frequencies will occur and the sound will tend to deemphasize some frequencies in the test tone. Set phase of the subwoofer to make the sound have the widest audible range of frequencies. It is impossible to do this alone. An assistant is required to flip the phase control while you listen for this to work correctly. It simply takes too long to walk up to your sub, flip the phase, and return to your seat. Instant A/B comparison is needed.
Lacking an assistant, you can use your SPL meter by positioning it at your normal head position. Note the SPL reading as the test tone is played in each phase. Select the phase with the higher reading as that is the one with the least overall cancellation of sound between the two speakers (main and subwoofer). If your sub has a variable phase control rather than a 180 degree switch, slowly adjust the phase to maximize the SPL reading. At that point the sub and main speaker are at best phase coherence.
If you note little or no change as phase is adjusted, make sure that the delay or distance settiings for all your speakers are correct. If they are grossly wrong, the system may be delaying one signal so far that it will never come into phase.
The choice of crossover frequency is often fixed at 80 Hz. Some pre/pro's or subs have an adjustable crossover. The warble test tones in AVIA help you find a good crossover point. The tests waver up and down in frequency but not in amplitude. They sweep through the crossover range and well below. The warble effect allows the signal to act as its own comparison level. Bass response is often spikey in a room with small changes in frequency making a large change in response. The warble lets you detect unevenness more easily. If you hear a big appearance/disappearance of the test tone, then the warble is sweeping through a frequency range with a large inequality in response. Play the warble tone and adjust the crossover freq to make overall response as flat as possible during the first (higher freq) portion of the test. That should be the portion which is affected by the crossover.
It may be necessary to redo phase after adjusting crossover frequency. The two controls will interact. Once crossover freq and phase are adjusted, go back and redo the subwoofer level adjustments. They will also be affected.
As you can see, AVIA was designed to accomplish a much more comprehensive subwoofer adjustment than previous calibration discs. The end result is better integration of the subwoofer with the main speakers and reduction of the "separate" subwoofer sound. Unfortunately, many users expect a quick and dirty method. You can indeed do that with another disc, but should you short thrift your hard earned equipment that way?. We chose to provide a more complete tool, and the end result is better subwoofer integration, but it takes time and forces users to discover what truly is happening to their bass.
I hope this helps people realize why we didn't oversimplify and duplicate the test tone on both the LFE and main channels at an arbitrary ratio. Doing things the way we did in AVIA requires greater understanding about your equipment, but ultimately yields more accurate calibration and enlightening information than other means.
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Guy Kuowww.ovationsw.comOvation Software, the Home of AVIA DVD[Edited last by Guy Kuo on September 06, 2001 at 02:46 PM]

Great information!
Now the burning question.
Are the low bass test signals on Avia compensated in any way for use with the Radio Shack SPL meter, as it is known to read on the low side for lower frequency signals?
Pete
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No. The only compensation is for the discrete LFE in the 6 channel pan. That is 10 dB down electrically.
It would have been undesirable to alter the tones to specfically compensate for a single model of SPL meter. It would have made a mess for any other equipment because that poor sould would have to undo the compensation built into a signal for the RS meter and then applying the correction for the meter being used. Far better to keep it standard and only expose the users to a single layer of compensations.
The signals are kept at electrical equal energy and you should compensate for your particular SPL meter. That means on the RS meter making the SPL reading equal 85 dB to match the 85 dB on the main tests yields a sub which is actually a few dB louder than neutral. Since so many of us HT fans like a lot of LF effects and don't listen at full reference volume, this yields an often more pleasing effect than a fully flat system.
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Guy Kuowww.ovationsw.comOvation Software, the Home of AVIA DVD

Guy, thanks for the detailed explaination. It emphasizes the importance of subwoofer phase. Which leads me to this question (for Guy or anybody who knows)...
My subwoofer doesn't have any phase switch whatsoever. Would reversing the +/- speaker wire connections for ALL 5 speakers at EITHER the speakers OR the receiver accomplish the same cumulative effect as reversing the phase on the sub? Granted, it makes an A/B test difficult...
Thanks!
JKS
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Yes, that would have the same effect with a few caveats. The total system phase will be inverted, but since absolute phase of a recording isn't maintained the same from recording to recording, it's a moot issue. Of more concern is accidentally missing one of the speaker wire pairs.
Is the sub fed via an RCA jack or through speaker wires. If the latter, you could invert the speaker wires to the sub. I've tried inverting an RCA jack connection, but the resultant AC hum wasn't acceptable.

Gary, truth be told I couldn't really tell you. The 7.1 processing takes the original 5.1 channels and redirects things. All I can say is that what you hear it doing to the test tones in AVIA are what it is doing to actual material. Beyond that I'd have to have a unit here to examine how their steering logic works as the signals flip phase in the test tones.

Guy,
Great info. It confirms what I've heard about phase really being a 2 person job. It didn't occur to me that changing phase would change SPL's though. I do think phase can be done by 1 person with an SPL meter, a tripod and a pair of binoculars though.
Is listening for the widest range of signals when setting phase essentially the same as listening/measuring for the loudest SPL's?
On the RS meter's inaccuracy should I be going with 2/3 db lower than the other channels or more? I want to achieve as flat a bass response as possible. That extra "couple" of db's is good for movies but out of place with music.

Great Post!
I took great interest in Guy's comments about users enjoying higher bass levels at lower overall volume settings. I found that when I calibrate my sub to 85 dB, to match my main channels, the overall perceived bass level seemed weak when listening to music or films at less than reference level (in my home, this is most of the time). I found that I had to dial in about a 10 dB boost to the sub to achieve what I considered adequate bass levels at most normal listening volumes.
I found an interesting article at the following link: http://www.line6.com/Main/The_Lup/Ti...s_Tutorial.htm
It explains the concept of loudness, and how our perception of loudness varies with frequency and overall level. The article pertains to guitar amps, but the concepts apply to any sound system.
None of this is new (the studies were done in the 1930's!), but it went a long way in explaining why I personally prefer some amount of bass boost, as opposed to dialing in the system (via SPL measurements and EQ) to be flat (or as flat as possible in a normal room).
If I might pose question or two:
What are the frequencies of the alternating AVIA test tones that are used to match the sub level to the main channels?
And secondly, would those tones only be accurate if the crossover frequency were within a certain range?
I run a fairly low crossover (about 52 Hz or so) to work with my B&W 604's and HSU 1225 sub. This produces a subjectively and measurably smooth transition in my room, but I always suspected that with if the level match tones assumed a higher crossover, I may be getting too much sound out of the mains when playing the low frequencies that are intended to be primarily for the sub.
Best Regards,
Brian Leduc

The subwoofer test signals are designed to work best with a THX spec crossover or slightly higher.
The lower portion of the tests is bandpass from 35-70 Hz (one octave) and the upper is 500 to 2kHz (two octaves). However, the rms levels of the two portions of the tests are identical so they will read the same level on your meter. Both bandass filters are brickwalled at something like 14th order.
If your crosssover is lower than 70 Hz, some of the low pass portion of the signal will end up going to the mains instead of the sub. That does change things in terms of how much of the tone is reproduced by the sub vs mains, but it doesn't totally invalidate the tests. These are not tests of what proportion of a signal should go to a sub, but of how flat frequency response is when you compare the midband and lowband. They should be equal in a properly balanced system. You are setting flatness of the lower end system frequency response by using these tests. You can only do this by turning the sub gain(s) up and down. The system still needs to reproduce both parts of the tone at equal level so if things are flat in response regarding those two pass bands no matter where your crossover point lies.
A super low crossover means that you can control the very very low frequencies, but lose the ability to fine tune the response of the system at bass frequencies above your chosen crossover. This is a tradeoff which only you can pass judgement.
Actually, while we are at it. The AVIA tones are actually well designed for the subwoofer over-endowed. If one has separate sub for each channel, the fact that each channel has an independent subwoofer setup tone means you can set each channel to be flat.
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Guy Kuowww.ovationsw.comOvation Software, the Home of AVIA DVD